With a 16:9 screen, 10" of diagonal for each foot of viewing distance is considered about average. That is, most people would have no problem viewing a 120" screen from 12'. Compared with a commercial cinema, that represents about the equivalent field of vision of sitting toward the back. Those who like to sit more toward the front of a commercial theater would want a larger than 120" screen when viewing from 12'.
The smart way to determine the best screen size before investing in one is to buy the projector first and project on a blank wall. Try watching a variety of content on a variety of image sizes until you find the biggest size you can watch without getting eyestrain. Then just measure the diagonal of that image and order a screen of that size. Just be sure the projector you buy has a throw range and zoom range to be able to project the range of image sizes that you think you might be likely to consider.